Cascaded multi-phase electric drives are used in industry to provide variable electric power to AC motors. These same drives can be used in other applications not related to specifically to motors but where a variable-output voltage or frequency is desired. Typical drives have an AC input power source and some type of conversion apparatus, usually using solid-state devices, for converting the fixed AC input voltage into a variable-voltage and/or variable-frequency output. One such type of drive is described in U.S. Pat. No. 5,625,545, which is incorporated herein by reference. That patent describes a power supply used as a drive which utilizes a number of power cells (power units) arranged to produce a three-phase AC output. Such multiple power units in series can be utilized to provide higher voltage outputs than would be available with only a single power unit.
In a conventional cascaded multi-phase drive, electric power fed from the grid flows to the load through the primary windings, the secondary windings, and the cascaded power units. Usually fuses are assembled on the power units. Where there is a fault at the secondary side of the drive, for example short circuit current leading through a path including a secondary winding, fuses and power units, the fuses get blown first and the short circuit current path is opened and thus the other components in the path are protected; however, if there is a short circuit between phases of a secondary winding and the short circuit loop does not include the fuse, then it will not be opened until at least one phase of the secondary winding is broken to open circuit; in other words, the transformer cannot be protected. Besides, where there is a fault at the primary side of the drive, its control system identifies a fault on its components by measuring its DC-link voltages of the power units or primary transformer currents; however, this hardly works where the load is light or no load; in addition, if there is a low voltage ride through at the grid side, it will take some time when such fault is reflected on change of the DC-link voltage of a power unit, and thus its identification is delayed.